text:
Why does a stall decrease lift, rather than increasing it?
A stall occurs when the angle of attack of a wing or other airfoil becomes so high that the airflow over the upper surface of the wing separates from the wing, rather than remaining attached to it; this causes the wing to produce less lift and more drag, making it harder to maintain level flight and more sluggish to respond to control inputs.
However, shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing, and, thus, greatly increase lift?  What am I missing?

I suspect (but don't know, which is why I'm commenting and not answering) that the salient point is that the airflow is separated, meaning it doesn't follow the nice contours it should -- but it's not a vacuum. So you still have air (and thus pressure), it's just not doing what you want it to.

For a parcel of air to generate a lift force as it flows over the wing requires the wing to tip that air parcel's momentum vector downwards slightly; the reaction force that the wing experiences as it does this is what we measure as lift.
In the case where the airflow over the top of the wing separates from it, the parcels of air flowing by do not get their momentum vectors redirected downwards anymore and the wing hence "stops flying".
Meanwhile, the region of separated flow constitutes a zone of turbulent air which forms a stirred-up wake in the rear-facing "shadow" of the wing at its high angle of attack and the only work performed by the wing in this case is to stir up that wake- and that constitutes lots of drag.

Long ago I read an article where the writer demonstrated that the package of air redirected extended about half a span above the wing and did calculations that showed quite a staggering amount of air mass redirection was involved in generating action/reaction force.  It was the low pressure distribution created by the airfoil curvature that induced air above to move down so the wing was doing more than simply deflecting downward the air directly below.  The stall disrupts this "suction induced" air movement so that only the air below is being deflected, killing off most of the lifting force.

Because an aircraft in level flight is in equilibrium with gravity, a sudden reduction in lift can have a dramatic effect.  A rapid sink and nasty shuddering as stall turbulence rocks the aircraft.  However, checking the lift to AOA curve shows the plane still has significant lift in stall, but not enough to match gravity.  Exacerbating the situation, the high AOA lift vector now has a rearward component, essentially "putting the brakes" on forward motion. AOA increases as you sink, moving towards deep stall.  Fortunately, as you sink, the (rear) H stab will try to push the nose down.

@RobertDiGiovanni: It only moves towards a deep stall if your aircraft has a T-tail; deep stalls are physically impossible in aircraft with conventional tails.

When a plane climbs it does so against the flow of air through the wing, and it changes altitude while crossing through different air pressures and air in different directions, hence airplanes slow down during lift (relatively).
You do forget to consider the air that resists the fuselage as well.
As @MichaelHall below mentions, A stall only occurs when the critical angle of attack is exceeded. This can occur in a climb, but it can also occur straight and level, in a turn, or while descending.
A Stall Generally means the plane's wings are getting reduced Air flow on them to maintain the stability in speeds. It can also occur when it climbs too steeply with the wing getting resisted more during lift. This is when the airplane loses lift or it loses the ability to maintain the plane on the air:

Source, modified by me

Source

Nice pictures, but your explanation is not clear.  For example:  "A Stall occurs when it climbs too steeply." is not correct.  A stall only occurs when the critical angle of attack is exceeded.  This can occur in a climb, but it can also occur straight and level, in a turn, or while descending.

The horizontal stab is in the turbulent flow.  Is this jet's bad day about to get much, much worse?

@WayneConrad It can actually happen, is called "deep stall" and this is not a good thing. In fact if the turbulent air is in the path of the control surfaces, the controls are not working anymore so you cannot get out of the stall and yes, that means you should think over the question: "Did I update my last will?"

@ThorstenS: Technically, it is possible to escape from a deep stall by rolling to a high enough bank angle that the horizontal tail leaves the "shadow" of the wings (since you still have at least some aileron control authority).  Still a very bad day for any occupant(s) of that plane, though.

Thank you, i have added the source. @MichaelHall True, i forgot to mention that part, and it is because the aircraft is not able to generate enough lift right. It may be because of the engine and sometimes due to the pilot . . .
Sean, it may be possible, however in the pilot's manual it is suggested to drop your nose down to generate enough speed to get lift again . . .

@MichaelHall I've your explanation to my paragraph, if that is alright

yes, much better. I'll finish fixing that for you. Thank you.

@Ani: ...and how, exactly, do you drop your nose down when your elevators and horizontal stabber are completely ineffective?

Yeah, right that! @Sean

First sentence could be clarified.  Are you trying to describe something that would not happen in completely still air?  A discussion of climbing up through a wind gradient really ought to be a completely different topic.  Headwind increasing or tailwind increasing?  "Slows down" in terms of airspeed or groundspeed? Is the effect temporary or lasting as long the "during lift" lasts?  Does "during lift" mean climbing, entering an updraft, or just pitching up to an extreme-of-attack?  Now I'm suspecting the  latter but if so it must be stated more clearly. Many opportunities to clarify here.

However, shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing, and, thus, greatly increase lift? What am I missing?

Well the total aerodynamic force DOES indeed increase, but as it is now pointing almost backwards most of it is decomposed as drag, and virtually none of it is left as lift.
By def. the rearwards component of the total aerodynamic force is drag.

It is possible the question is not being answered adequately because the question is incomplete. perhaps the continuation to the question would be,"does the pressure differential generate the lift or does the displacement of a mass of air caused by the differential in pressure of a wing in motion thru free air generate lift?"
Implying that reduced pressure above the wing is generating lift is confusing an effect of a result with the result of an effect. Great theory, not experimentally proveable. NASA tried during boundary layer research on a B-66 airframe in the 1960s. worked well providing BLC but lift? The data said no. Fluid dynamics may have the answers but be forewarned, it will be very complicated.
One does not expect to see an airframe levitate off the ramp by evacuating the mass of air above a wing surface and expect the resultant pressure differential in free air to "push" the airframe vertically upwards into the air, if only it were so. NASA did prove that holes in the wing BLC surface ducting smaller than the dust particles suspended in the ambient air showed a proclivity towards being clogged by said particles thus reducing BLC effectiveness; causing NASA researchers to move on to experiments in blown flaps. That, so far as my researches has taken me over the years, is about as close as "experimental data" has gotten to answering your question.
But I digress.
at stall, the air below a given wing is still at a higher pressure than the air above the wing and seeks the low pressure area by reversing flow above the sharp TE of the wing to "fill" the "void". All the data about lift vectors changing during stall conditions is quite true and proveable. Many theories of how wings generate lift are all seemingly correct. one needs to consider them all and will yet will still conclude that, at least in this universe, Newton is still correct. For every action there is an equal and opposite reaction.
So, stick to the official approved "doctrine" when answering test questions or oral examiners questions in order to pass the required testing. Remain open to new ideas from young minds, that is there job. Newton also commented on laws of inertia that many people hadn't considered, "inertia increases exponentially with the size of the bureaucracy".

Hi and welcome to the site! Could you edit this so it is less of a wall of text and more readable?

@AEhere The author of this post mistakenly used single line breaks instead of paragraph breaks; I've edited the post to fix those.

@TannerSwett Thanks for the fix.

Sadly this does not appear to answer the question at hand, and contains several inaccuracies as well, like the bit about Newton (not universally applicable, breaks down near the relativistic limit).

Also, I'm not (and wasn't when I asked the question, either) studying for any test or examination.  I'm just curious.

@AHere I think it does answer the question, it actually says pretty much the same thing as the accepted answer. Also, why do you think Newton's third law would no longer hold in the relativistic limit (not that aircraft are remotely relativistic anyway...)?

@Bianfable I meant in the general sense, Newton's laws are not universal, and the better known limit is their failure to describe relativistic phenomena. The third law in particular does not hold for electromagnetic forces, unless I am mistaken: https://physics.stackexchange.com/questions/35302/when-does-not-newtons-3rd-law-apply

@Bianfable I guess we disagree on the answer part. I can see some elements of the accepted answer, but they are scattered among boundary layer control experiments and allusions to "approved doctrine".

There is no more "large area of low pressure above the wing" in the stall. On the opposite, stall is when the "void" on top of the wing sucks in the air from the beneath the wing through the area of less resistance: around rear edge. It releases the pressure between over and under- the wing drops. One should NOT! be thinking about Bernoulli principles when concept is same as explaining the lightning strike: critical imbalance of negative and positive currents (or pressure in our case) leads to a dramatic discharge when opportunity arise.
Seeing is believing: lots of YouTube videos show people attach stripes to the wings and fly them to stall visualising the air currents. I.e.

You'll see how the story happens: in the beginning all is normal and the stripes are aligned with the headwind ok. Then another current starts from the wing rear side and pushing the stripes around. The two currents ARE fighting for the wing. You can see the headwind is loosing the combat as the battleground (see those chaotic stripes) moving forward and all the stripes behind it are turned the opposite way, not because of a tailwind (the aircraft is not flying backwards!),but because we have a lot of air escaping high pressure zone beneath the wing and filling low pressure zone over the wing. And it goes from around the back of the wing. The lift destroyed.

Hi and welcome to the site. While the video is a good resource, your explanation does not address stall modes where the stagnation point remains at or very near to, the trailing edge. Recirculation around the trailing edge is not a requirement for stall, iirc.

Can you find a video showing the "modes" you referring to?

Not a video, but it serves my point: https://ars.els-cdn.com/content/image/1-s2.0-S0360544217320789-egi1073XKD7317.jpg

More specifically, my point is that there is no need for the flow under the wing to move into the area above it. The flailing strings in your video are more likely to be caused by the shed vortices above the wing than by airflow coming around the trailing edge. Your explanation is easier to visualize and understand on an intuitive level, but not correct for the general stall case.

@AEhere "More specifically" there IS the need for air or fluid to travel from high to low pressure zones. God created the need and physics described it. The "more likely" meaning you're fantasising. So you are coming up with your theory, but as long as it is useful to others and explains some cause and effect it means nothing. Yours is nonsense because it needs a miracle to counterbalance the lift. Same as Radu094 who implies that at the 16 degree pitch wing reactive force directed mostly backwards. Sorry, if you don't like my theory, just ignore me.

@AEhere Also, your link is pointing to the snapshots not relevant. This thread is discussing the real world situation: the wing is moving and the air is static. Your snapshots are taken in laboratory simulating the opposite: static wing and kinetic air. Air has inertia, the mass, you know, therefore the picture will always be different in this simulation: namely the dark area is elongated behind the wing. The more accurate simulation would be to move a spoon in a paint bucket ;)

I am sorry you prefer theological arguments over experimental evidence. Do the scriptures provide a rationale for why relative motion has a privileged frame of reference, so moving air over a wing is different from a wing moving through air? Hint: it doesn't, but feel free to believe.

I did not believe it before either. But it's off the topic. So, what is your answer to the thread question?

@ymb1 I am new here and not allowed to comment directly to your answer. My car does not have a very aerodynamic shape at the rear, so the separation happens there all the time. How do I not get the "stalling" forward sensation? ... Separation + Vortex = side effects, not the causation. Also, speaking about the wing, are you sure the top of the wing faster air current produces the low pressure and not other way around? I think your explanation follows the doctrine and doesn't  answer the question above.

I have not written an answer to this question, so you must be confusing me for someone else; the accepted answer seems good to me. If you have a question about car aerodynamics you can pose it separately, though. You may also want to review this question regarding stalls: https://aviation.stackexchange.com/questions/66185/is-there-a-standard-definition-of-the-stall-phenomena

Starting off with a little physics: An airfoil (wing) generates lift due to pressure variations created by the shape of the wing.  Bernoulli's equation which models the laminate flow of a fluid (or in this case a gas) though a restriction is: P+(rho*V^2)/2 = n  (Pressure + (density x Velocity squared) divided by 2 = some constant.  I'll spare you the entire derivation but the end result is the equation for lift which is:
Lift = CI * (rho*V^2)/2 * A
Where:

CI = Coefficient of lift
rho = Air density
V = Air Velocity
A = Area of the wing

In essence this shows that the faster air is forced over the top of the wing, the lower the air pressure becomes above the wing.  Since the air pressure under the wing remains high due to a relatively flat surface, and that nature will look to fill a void, the wing is sucked up into the low pressure (or, if it is easier to wrap your head around the high pressure below the wing pushes it up into the low pressure above the wing (Newtonian view)) thus, we have lift.
(By the way, the Newtonian view and the Bernoulli's view are not mutually exclusive, they are two different approaches that show the same thing... but I digress and that is a whole other ball of wax)
Now, to the meat of your question:  As we increase the angle of attack(AOA), lift increases due to a higher coefficient of lift.  This would seem to suggest that your initial supposition is correct after all, higher AOA mean greater coefficient of lift but if we look back at our equation we see that the velocity of the airflow over the wing is critical to lift.  If we take an idealized look at the airflow as it separates from the wing(Critical Angle of Attack), the air velocity over the wing drops to zero which means we have: Lift = CI((rho*0.0)/2)*A = 0.0, no lift which causes a stall.
As to the second part of your question, why does the drag increase?  A fairly straight forward application of Newton's laws tells us what is going on.  As the AOA increases and increasing large surface area of the wing is exposed to the relative wind.  Combine that with increasing vorticities behind the trailing edge of your wing, creating a low pressure area, and you have increasing drag.
Hope that helps and best of luck in your flying.

The lower surface of a wing is not necessarily flatter than the upper surface. Many aircraft types have symmetric airfoils, and it's possible to create lift with a completely flat airfoil, such as a kite. You might like to read our question on how wings generate lift. I think to really explain the stall you need to focus on why the flow separates.

You are correct, there are symmetric airfoils, there are also any number of variations on wing designs, swept wing, delta wing, rectangular wing, tapered wing, rotary wings.  Supersonic aerodynamics is a good time and of course, a wing will still 'fly' even when inverted.  None of that has anything to do with the price of tea in China.. Bernoulli's principle still holds (the magic happens in that CI variable).  Have a gander at NASA, MIT Aeronautics or Embry Riddle's webpages.  I'll spare you the alphabet soup associated with my name, it's meaningless on the web anyway.

Air decreasing pressure above the wing has nothing to do with the speed of the air, it is actually due to inertia. Air always wants to fill empty spaces due to the fact that there is something called static air pressure. All air molecules want to push equally in all directions so when there is a space without air, the molecules fill the space due to there being no air molecules in the space to push against them. This is what happens with a wing, the molecules want to fill the space above the wing but they have trouble doing that due to their inertia.
Think about a car turning, you can't just go 100 mph and turn the in the same radius that you do when your going 10 mph. The same principle applies to the air molecules. They have a hard time filling the space and therefore the space has less air molecules then the atmospheric pressure. The less molecules on top means less static pressure on the top then on the bottom where the air is at atmospheric pressure. This forces the wing up due to the difference in force, so basically what lifts a wing up is suction.
If you want to say that air has a further distance to travel over the top and therefore goes faster, there is a simple way to prove that wrong. A completely flat surface can create lift as long as you tilt it relative to the airflow. The air has to travel the same length over the top and the bottom yet it still creates lift.
For stalls I'm not sure but I have a theory that in a stall, there's turbulent flow due to the air from the bottom trying to reach the top and vice versa but this only occurs behind the wing surface. Over the wing surface the air completely stops moving and just stays attached to the wing and therefore reaches atmospheric pressure.
This would explain how in a stall a wing both produces more drag and less lift because the equalization of pressures on both surfaces of the wing cancels the lift but there is still low pressure air behind the wing which sucks it back acting as drag.
Think about when your in a car and you open boot and put down the seats. Now the airflow at the back of the car will obviously be stalling but what about where you are sitting? The air feels normal there, it's not moving, it simply sticks with the car. Also when you look at a stalled wing in a wind tunnel with streams of smoke flowing by, you'll notice that there is no smoke streams right above the wing, just where the air is looping behind the wing. This is probably because the air isn't actually moving right above the wing, it's simply staying with the wing.
The stall thing is just a theory of mine but the thing about how inertia creates lift is the correct way that wings produce lift.

I do not quite concur with this. Yes, a flat plate will create lift if tilted properly w/r to the airflow. However, the air does not travel equal distance on top and bottom of the plate. This would require infinite acceleration from air particles as they pass the top front edge of the plate. The average trajectory of particles travelling over the plate is curved. As for the the car example, having driven a car without the top on, I can tell you there is one heck of a turbulence going on behind the windscreen... just opening the tailgate is does not resemble a wing, but a tube open at one end.


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Could reduced-lift wingtips allow an airplane to generate less wake turbulence?
Would it be possible to weaken the wake vortices generated by a large aircraft (which can be dangerous to smaller aircraft behind the large aircraft, and are the limiting factor determining how closely aircraft can be safely spaced in the air) by shaping the tips of its wings so that the lift generated (and, thus, the amount of air deflected downwards) by the outermost portion of the wing drops off gradually to zero, instead of abruptly transitioning at the wingtip from “lots of lift, lots of downward-deflected air” to “no lift, no downward deflection of air”?
Diagram of what I’ve got in mind:

This would come with a slight efficiency penalty (equivalent to slightly decreasing the length of the wing), but could still be useful in some situations despite this (such as high-frequency operations by a mix of heavy and less-heavy aircraft in heavily-congested airspace, or large numbers of takeoffs and landings by both heavy and light aircraft on closely-spaced parallel runways), and the reduction in lift generated by the wingtip would offset some of this penalty by reducing the upwards bending force on the outer portions of the wing (and thereby reducing the amount of material needed for stiffening this part of the wing).

By the way, I have seen a string (roughly 6" long) attached to the trailing edge of a wingtip spin in the opposite direction to what we would normally expect.  The aircraft had a great deal of washout and was flying at a high speed (low angle-of-attack).  The wings were supported by upper and low wires running out to midspan, and the lower wires were slack and the upper wires were loaded-- clearly the bending load at the root was not in the direction we normally expect.  Undoubtedly though at some distance further behind, a pair of wake vortices did exist, spinning in the normal direction.

Oops thought I deleted that comment when I posted an answer.  Too late now--

Lift already does taper off gradually towards the tips, even on rectangular wings without any washout. That effect becomes stronger with tapered wings, of course. The reason is the pressure equalization resulting from flow around the wingtips, like you show in the lower picture. And like in that picture, the wake is spread over span in reality. Your upper picture is unrealistic and a gross oversimplification.
However, the eventual strength of the wing wake will be little affected by this. The resulting vortices are caused by wake rollup and do not come directly from the tips. The tip vortices are only a small part of the overall wake.
Regarding efficiency: Tapering lift towards the tips will make the wing more efficient. Lift far away from the wing root causes high root bending moments and requires a heavier structure. Lift near the root costs less structural weight and the net lift of the wing (lift minus wing weight) will be higher with a near-triangular lift distribution. This insight was published as early as 1933 by Ludwig Prandtl and rediscovered by R. T. Jones in 1950.

I have seen a string (roughly 6" long) attached to the trailing edge of a wingtip spin in the opposite direction to what we would normally expect. The aircraft had a great deal of washout and was flying at a high speed (low angle-of-attack). The wings were supported by upper and low wires running out to midspan, and the lower wires were slack and the upper wires were loaded-- clearly the bending load at the root was not in the direction we normally expect. Undoubtedly though, at some distance further behind the aircraft, a pair of wake vortices did exist, spinning in the normal direction, despite the fact that the areas near the wingtips were clearly creating downward lift.  This isn't as relevant an answer as the one already posted but it does bear on your question to some extent.

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Why we had two empty rows on busy Ryanair flight Boeing 737-800 and no one was allowed to seat there?
Back in 2012 I flew from Tenerife South airport to London Stansted with Ryanair airline. All the tickets for this flight were sold out. But 3rd and 4th row on this Boeing 737-800 were empty (12 empty seats) 1 & 2 row are for customers who paid for the seats. The interesting think that no one was allowed to seat there. One lady before departure occupied one of the empty seat in the 4th row. Cabin Crews told her that aircraft will not depart unless she will go back to her seat. At first lady refused to go back and started complaining. We were waiting for her to change the seat and then we departed.
I did ask the steward why is there two empty rows but had no clear explanation from him

I'm voting to close this question as off-topic because any answer is purely conjecture; there is no "correct" answer.

@Uraflight  Travel Stack Exchange is usually the better place for these kinds of questions.  Most users here are pilots, aerospace geeks, and technicians, not flight attendants/stewards or gate agents.  A closely related (albeit not duplicate) question is here: https://travel.stackexchange.com/questions/58190/ , and you may find the following questions also helpful https://travel.stackexchange.com/questions/48827/ or https://travel.stackexchange.com/questions/14828/

@abelenky: There must be a correct answer from Ryanair.

@CodyP While I agree this is pure conjecture at this point, there are aerospace related reasons, specifically weight and balance. This question may be relevant to the OP: Are passengers really not allowed to change seats before take-off due to weight and balance?

Were these emergency exit rows?
Seat pitch on low cost airlines is too small to allow an emergency evacuation within the required 90 seconds with regular seat pitch in the exit rows. Therefore, the seat pitch is larger there, which makes a world of a difference for tall people. Low cost airlines have decided to monetize those few more inches of leg room and charge extra for sitting there.
If no passenger on that flight was willing to pay the extra fee, those airlines which pride themselves on treating their passengers with the utmost contempt keep these emergency exit row seats free even if the plane is fully booked on all other seats.

I'm voting for this, though the exit rows are 16 and 17.  Seatguru shows the Ryanair aircraft have designated rows 1-5 as premium seats with extra legroom and priority boarding.  Those seats will go empty if no one pays the extra €17/£15.  It may have been only rows 1-4 were premium in 2012, and the cost could have been more.

They were 3rd and 4th row . The flight was operated in 2012. At that time only 1 and 2 row were a premium seats

Beyond that, their Flight Attendants are probably told, when we tell you that these seats are blocked off, keep them blocked off regardless. Even if there was no valid reason that those seats needed to be the ones blocked off (say, if total weight was the issue; balance ISN'T it on a full 737-800), the FA's probably don't have the discretion to say "today, you can change seats", since tomorrow, you might have to be told that you can't. And it's easier to make blanket rules than to let people think & adapt to the situation.

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Why can't some airports handle heavy aircraft while others do it easily (same runway length)?
As I've seen in this video, they operate flights from Bournemouth airport all the way to Barbados on a Boeing 787-8. As Bournemouth runway length is 2,271 m (7,451 ft), it can handle aircraft such as Boeing 787 fully loaded.
In Kiev Airport (Zhuliany) in Ukraine, runway length is 2,310 m (7,579 ft). But the maximum it could handle is an Airbus A320.
Airport authorities keep telling us that they need more money to extend the runway length. And after that they will be able to handle larger airplanes such as the Airbus A321, but still not as big as the Boeing 787.
Another city is Kherson (Ukraine), the runway is 2,500 m long and can only handle the Airbus A321.
Are the local authorities misleading the passengers? They keep telling us that they need more investment/money. It looks like they are lying to us.

Anecdote: I once read that they had to reinforce the apron stands at Frankfurt for the then new A380 by digging 6 meters (20 ft) deep and then filling the complete hole with concrete just to allow for her massive weight and prevent her from sinking in. So it's not only the the runway length that matters.

Another factor, at least theoretically, is altitude.

@Roger Altitude is a good point that should be included in answers or the question. These three airports are about the same altitude so it's probably better just to include it in the question.

Possible duplicate of "the freeways can handle fully loaded semi-trucks. Why are they wrecking my driveway?"

It's always possible that somebody somewhere is (over-)simplifying the situation in good faith. Maybe the airport people said "work on the runway" because they don't want to confuse the council/the press with technical details about concrete and asphalt, and then somebody in the council/the press thought "well, what else is there to change about a runway other than its length", and that's how it got into the newspaper.

There are a few possible reasons Bournemouth (EGHH) can support a Boeing 787 but Kherson (UKOH) can't. Runway length is definitely not the issue. The runway at EGHH is 2271 metres long, the one at UKOH is 2500 metres long.
The runway at UKOH is slightly narrower, though: 42 metres compared to 46 metres at EGHH. Whether this has operational significance, I do not know.
Another thing to compare is the strength of the pavement, which is expressed as a Pavement Classification Number, or PCN.
The PCN of the runway at EGHH is 46/F/B/X/U, at UKOH it is 28/R/C/X/T. Those values can be decoded like this:

EGHH: Load carrying capacity: 46, flexible pavement (typically asphalt), high strength, high tire pressure supported.
UKOH: Load carrying capacity 28, rigid pavement (typically concrete), low strength, high tire pressure supported.

This is a bit technical, but what it means, in essence, is that the pavement at UKOH is not nearly as strong as that at EGHH. The 787 has a max takeoff mass of around 230 tons, so it needs some pretty strong pavement to support it.
Finally, the rescue and fire fighting services provided at the two airports is different. EGHH provides up to CAT 9 (capable of handling aircraft up to 76 metres long and 7 metres wide (fuselage width)). UKOH provides only CAT 7 (capable of handling aircraft up to 49 metres long and 5 metres wide.
The 787-8 (the smallest of the 787 variants) is 56,7 metres long and 5,77 metres wide. So most likely, what's keeping Kherson Airport from handling 787's is a big investment in new pavement, and maybe the need to buy a few extra fire trucks.
Then of course, there's also the whole question about capacity of the terminal building, size of gates and various equipment (push back trucks, stairs, jet bridges etc.). Those things are a bit harder to compare directly, though.

Thank you for your excellent explanation J. Hougaard. But what about Kyiv International Airport (Zhuliany) (IATA: IEV, ICAO: UKKK) ? They keep telling people on the news that runway length is the issue . I don't think there problem with a Pavement Classification Number, or PCN as runway has been refurbished few years ago

The PCN at UKKK is 46/R/C/X/T, which to my untrained eye seems like it is quite similar to EGHH. Runway dimensions are almost exactly the same. They only offer CAT 7 fire and rescue though, which is not enough. It's quite possible that there are other underlying issues as well, but I don't know.

+1. To add a few points: 1. Bigger airplanes can mean a lot of passengers at the same time boarding/arriving and the terminal must handle that peak of traffic. 2. Some terrain, like sand or mud, can make difficult and expensive to build strong enough foundations. 3. Neighborhood and environmental impact: Sometimes there are regulations demanding low noise level and that can affect types of aircraft can or cannot arrive or depart. Also, regulations can demand a safe distance from your neighbors and in order to add 300ft to the strip, you must first buy houses and relocate people.

Probably you meant "flexible payment" -> "flexible pavement". I tried to suggest an edit, but it needs at least 10 χαρακτήρες :/

@gsamaras Of course ;) Thanks to whoever did edit it.

Another potential factor is that a hotter, higher airport will need a longer runway to support the same plane because of decreased air density. (This isn't relevant to Kherson vs Bournemouth, since both are basically at sea level; Kherson is a bit hotter but not hot.)

Kherson seems to be about 15deg C  hotter during summer, that is a substantial difference,  translating to more than 1500 ft difference in density altitude.

How/where do you lookup PCNs? I was searching for PCNs for LGRX a little while ago and couldn't find it.

@jcaron They are listed in the textual aerodrome information in the AIP (AD 2)

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Can Airbus A400M take-off from the beach fully loaded?
I've recently saw this footage. Airbus A400M tactical capabilities on natural surfaces by performing take-off, landing and taxiing manoeuvres. I wonder if she could take-off fully loaded from that surface ?

I would think this highly depends on the type of sand on the beach...

it would. How compacted is it, is it wet or dry, what is the grain size, is there clay in it, rocks, etc. etc. all play a part. If it's compacted, somewhat wet maybe, a beach can be a good surface. If it's deep fluffy sand, or muddy clay, everything gets stuck in it, and you're not going to pull an aircraft through it.

I think answer can focus on known sand runways such as Traigh Mhor. That way, sand characteristics are known.

Generally an aircraft can take off heavier than it can land, so it probably could. However, most recent tactical landings for A400M occur (at least for french ones) in sub-saharian regions or Afghanistan where "hot & high" conditions may alter the overall aicraft performance more than the terrain itself. For instance heavy takeoffs are generally operated in the early morning or even at night to have better lift and engine performance than in the afternoon

The surface of the beach will need a certain California Bearing Ratio to be allowed to have the aircraft land there. This will be measured by the ground support prior to the arrival of the aircraft.
This will allows the aircraft to operate largely normally when on the ground roll and taxiing, however the crew would need to be mindful about brownout when using reverse thrust and to be gentle when turning so as not to bury the nose wheel.
The aircraft will have a maximum mass for operations at the given CBR. There will also be a max mass based on the aircraft performance versus the field conditions and parameters with margins for safety (though not the same as standard Engine-Out performance or Balanced Field) which is unlikely to be its normal maximum takeoff mass.
EDITED for scope.

Points 1 & 3 seem to be mainly focused on landing, while point 2 addresses the actual question about takeoff. You may want to edit to focus on the takeoff portion.

The A400M has multiple missions. Each has a maximum payload.
Strategic airlift is to prepared surfaces and has a maximum payload of 37 tons.

Tactical airlift is directly to theater on rough or soft surfaces (CBR6) has a maximum payload of 25 tons, plus five tons of fuel for return. Return fuel is not needed for strategic airlift. It can do this 40 times before the surface is no longer usable. Typically, tactical is measured in how much can be delivered, not retrieved.

Maximum takeoff weight is 141 tons, which can be done according to the following chart from a soft surface if you have enough runway.

However, the soft surface penalty is seven tons for the above missions. You decide what 'fully loaded' means.

That would depend on the density altitude, consistency of the sand, length of the beach available, and the technique used for the takeoff. In addition to that, you'd have to be pretty much continuously moving during loading so the aircraft wouldn't sink into the sand. All things considered, I don't think there would be an issue if all these factors aligned.

Thank you for all your answers


text:
What range or variation should we expect from gyroscope in a helicopter?
I need to measure the angular variation of a helicopter. Hence, I am looking for a 3-axis gyroscope? However, what kind of specification the gyroscope should have, like sample rate etc? Therefore, I want to know how the angular velocity changes (in any axis) in helicopter. I need to know the range only.
Any reference would be highly appreciated.

Angular displacement due to flight manoeuvres?

@Koyovis yes, in addition while moving in land in wheels etc.

3-DoF magnetic gyroscopes are a dime a dozen nowadays, together with 3-DoF accelerometers, magnetic field sensors, and GPS position transducers. These are in mobile phones now, tiny and frugal with electricity. A google search for "9-dof position acceleration gyroscope" results in a lot of options.
For the sampling rate: this needs to be at least twice the frequency of the fastest vibration - more is better, since a frequency at half the sampling rate shows up as a triangular wave.

For the frequency range of angular displacement, this article might be of interest. It shows vibration data of a car driving over the road, using an iPhone to measure the vibrations. The article mentions a sprung frequency (inside the vehicle) of 1 - 5 Hz. A helicopter usually has higher inertia than a car, so sprung frequency should be lower. If you take 0 - 10 Hz as a baseline for the range, you should be safe. The ground reaction angular frequency range is always higher than the airborne one: responses in the hover are measured in seconds, not in Hz.

Above graph shows pitch response of an example helicopter, from Prouty, Helicopter Performance, Stability and Control. A helicopter with a rotor mounted on top is unstable in the hover, but in order to still be controllable for human pilots the response frequency may not be too high - it turns out that humans can comfortably control instabilities with a time period in the order of around 10 sec. The military have requirements for the stability response in MIL-H-8501A

"The ground reaction angular frequency range is always higher than the airborne one." Is there any reference or insight to it? This is the most important insights I was looking for.

Yes you're right, that is a bit of a sweeping statement. Have expanded the answer.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why would an airplane tip backwards during unloading?
I flew on JetBlue Flight 1011 BOS-ORD today, 2017-01-14. At the arrival gate, after 2/3 of the passengers had deplaned, a flight attendant asked us to stop and wait. He told us that the pilots were concerned that the plane was "tipping" to aft. I overheard the pilot say that the front landing gear was fully extended. The pilot also mentioned that they had "never seen anything  like this," that there was no cargo on board, and that the baggage handlers had 30-40 bags remaining at the rear of the plane. After about ten minutes, we were all allowed off.
Why would something like this happen?

BTW, looks like your flight was an Embraer ERJ-190.

Question similar to "Which Aircraft Models need a Tripod Jack for tail support at airport?" https://aviation.stackexchange.com/questions/24569/which-aircraft-models-need-a-tripod-jack-for-tail-support-at-airport

A picture is worth a thousand words... Passenger airplanes do not fasten baggage in the belly down, occasionally there can be a concern from the baggage pilling up at the tail (i.e. turbulence). New baggage employees may also have offloaded the aircraft wrong - unloading the front first.
I have been on airplanes where the flight attendants moved passengers from the rear of the airplane to first class because the aircraft was too tail heavy.
Most newer jets have pressure sensors on the gear. This allows the flight computer to show the crew how the aircraft is loaded in real time. The pilot may have received a warning the nose gear was too light or he may have notice the aircraft tipping up.

As I understood the OP she knew about forces on levers but could not understand why that happens to an almost empty aircraft....


text:
What is the typical voltage used on jet airliners?
I have seen several parts that use 28 Volts DC, is this a standard for jet airliners (e.g., Airbuses and Boeings).

Additionally you may want to know what do the generators/batteries initially deliver, before conversion.

You'll see all kinds of power on large aircraft - 28 VDC, 115 VAC 60 Hz (and likely 230 VAC 50 Hz), 115 VAC 400 Hz, 5 VDC (USB power in cockpic/pax seats), 230 VAC, and 270 VDC.
Here's a general idea of what the Boeing 787 Power Distribution System consists of:
(Source)

Airbus use 28V DC, largely for things like sensors etc., so the overall power usage on the DC system is quite low: As an example, the A350 has 4 Transformer Rectifier Units (TRUs) which convert AC to DC. 2 of them are designated as 'normal' and 2 as 'emergency'. Each one has a nominal power of about 300W. Total DC system usage is about 500W.
.
Unfortunately I don't know about Boeing.
EDIT: Was curious - looks like Boeing use 28V DC as well - which makes sense, I expect Airbus used 28V DC on the A300 as that was the standard at the time, and just kept using it until today.

28VDC was in use in aerospace applications already before the A300's introduction. For example, Apollo used 28VDC (see PDF pages 4, 6) and Apollo Spacecraft Familiarization training material for course APC-118 (see PDF page 152). At which point one would have to ask: Why not use what obviously works? Absent good reasons to change, one might just go with what's already used for similar purposes elsewhere in the industry.


text:
What were the effects of the oil crises on the Concorde ticket prices?
One of the things that is considered to have greatly hampered Concorde's success were the dramatic oil price increases due to the 1973 oil crisis (the 1979 one surely didn't help either, but it was too late already).
Wikipedia tells me that the 1973 oil crisis resulted in crude oil prices being multiplied by four. How much could this have affected Concorde's projected ticket prices ?
This is a very complex question, because I'm really interested in how much this affected the commercial case of Concorde, meaning a standard occupancy rate would need to be considered (ideally what was considered initially for Concorde if anyone has any idea...). But due to the difficulties of Concorde, the occupancy rate was usually very low (surely much lower than projected without the oil crisis), to the point that actual ticket prices surely ended up reflecting that. But that is a triggered consequence I would like to be able to distinguish from the original factor (after that we can of course add the calculation when the occupancy rate gets reduced as a result).
Basically :

Do you know what were the pre-crisis projected stabilized (ie non-initial) ticket prices of a given long range Concorde flight ? That would be immensely helpful ;
Do you know how much a 4x increase in crude oil prices at the time would have affected this projected ticket prices, all other things being equal (same occupancy rate, etc) ? If we knew typical jet fuel prices at the time (pre- and post-crisis), seems like an estimate of the additional "first degree" cost per ticket could be reached by dividing fuel capacity over expected number of passengers. Adding benefit margin and we would have an estimate of price increase.

Thanks.

The Concorde used one gallon per passenger per 17 miles of distance flown. Paris to New York is 3636 miles, so the price differential is 214 times the cost increase per gallon of fuel.

Thanks for the first bit of info (that's the cost differential though, some margin to be added for price). Now we'd need the $ price increase of jet fuel at the time and at least the second part of the question can be answered. I hope information for the first part is not going to be too hard to come by.

You should also take inflation into account when speaking of a price multiplication over several years. The equation may be quite complex (I like this question)

The Concorde started service only in 1976, so there never were actual pre-crisis prices. This makes your first question impossible to answer. Of course the airlines which ordered the Concorde had their plans how much to charge, but kept that a commercial secret.
When service started, the plane lost money. What happened next is an interesting lesson in economics. From A Pricing Lesson from the Concorde:

The Concorde team decided to do some market research. They asked
  businessmen how much they thought a Concorde ticket cost. The answer,
  “Most of them didn’t know. It was their secretaries or travel
  companies doing the bookings. When they were asked to guess, because
  they were senior, very important people, they all guessed that the
  fare was higher. ” – explained Captain Jock Lowe, Concorde resource &
  Planning Manager.
This insight led to a new pricing strategy. Captain Lowe described “So
  very simply, we said, we’ll charge them what they think they are
  paying. And so we put the fares up”.  There was a discrepancy between
  what the company was charging and the value customers saw in the
  product.
Concorde ticket prices were doubled to over $7,000,  one way, in
  today’s prices. As a result, Concorde was repositioned to provide a
  super-elite class for bankers, the rich, and the famous. Concorde
  became the place to be seen.

Generally, ticket prices were 1st class + 20%. So what affected ticket prices most was the all too human tendency to show off; the mechanism that makes luxury brands possible. Fuel became less significant.
Now let's look at the fuel cost. This table only starts in 1975 (but covers the full operational history of Concorde), and when I apply your factor of four, the pre-1973 price of jet fuel would had been 8 cent per Gallon. This means the nominal price has gone up by a factor of 50 when compared to the peak in 2008.

The Concorde typically needed 214 Gallons of fuel per passenger on the Paris - New York route (see my comment for evidence; using @mins much better source gives 236 Gallons), so that means fuel cost per ticket would look like the plot above, multiplied by 214 or 236. At the start of operations in 1976 that would had been about 70 USD per ticket, would had jumped to 200 USD per ticket at the start of 1981, would had dipped below 100 USD briefly in mid-1986 and again in 1998. The peak would had been above 800 USD in mid-2008 had the Concorde still been in operation at that time and would today be nearly 300 USD.

I know Concorde never started commercial flights pre-crisis. But it was built with pre-crisis prices in mind, and that's what I'm interested in. I fully agree this information would be very difficult to get (I wouldn't say impossible, as some people did have access to it at the time, so it's possible someone could share it).

Now, about the fuel costs, if we want to compare it to ticket prices, we have to make sure either both or neither are inflation compensated. The 7000$ figure for ticket prices is inflation compensated (ie in current dollars), but the 8 cent a gallon isn't (it's 0.08 of a 1975-dollar).

@ParkerLewis: Actual prices were all over the place. Just use the 1st class +20% for further calculation.


text:
When and why was AGGH's runway 06/24 shortened?
AGGH has one runway, 06/24.
When looking at an aerial photo of said runway, I noticed something interesting:

(Image by Graeme Bartlett at Wikimedia Commons.)
My interest was piqued by seeing what looked suspiciously like a dilapidated additional length of runway, so I went into Google Maps to take a closer look, and it seems that the thing of interest is indeed a length of runway in a state of considerable disrepair.  A set of old threshold markings (including the "06") is clearly visible, as well as what appears to be a displaced-threshold chevron.  Right at the end of the pavement, there's what looks like a VOR station:1

Runway 06/24 at AGGH is currently 7218 feet (2200 meters) long.  However, if the approximately 3000 feet (915 meters or so) of dilapidated runway to the west-southwest of the current arrival end of runway 06 were included, the runway would be over 10,000 feet long and able to safely accommodate considerably-larger aircraft:

It appears that runway 06/24 was shortened significantly at some point for some reason.  When and why?

1: It looks like the VOR station (if, indeed, that's what it is) was most likely a later addition taking advantage of the shortened runway (as opposed to having been the original reason for shortening of same), given that, in the first image, the disused segment of runway is already visibly-considerably-degraded, indicating that that photo was taken after said runway segment had already been out of use and no-longer-maintained for a considerable time, but there is no trace of the VOR(?), nor even any signs of construction work at its location, indicating that the looks-like-a-VOR was built there long after that stretch of the runway'd been abandoned.2
2: Besides, putting in a new VOR would hardly, in itself, have necessitated removing part of the runway, since there's lots of empty space elsewhere on the airport grounds where one could've been put.

https://ww2db.com/facility/Henderson_Field/ suggests the runway was expanded to the "current length of 2200 meters" in the 1970s.   Meanwhile http://usafunithistory.com/PDF/20-29/29%20AIR%20SERVICE%20GP.pdf suggests the runway was lengthened to 7400 feet during the war.  Haven't found anything about shortening or the abandoned section.

https://www.nzsappers.org.nz/wp-content/uploads/2018/11/1969-June.pdf has lots of neat descriptions, but suggests the field was only 5400 feet in the 1950s.

A little bit of sleuthing based on comments and the aerial photograph reveals one or two roadways that may have existed near Honiera International Airport before the runway was lengthened more to the northeast, remain as remnant "intersections to nowhere", particularly on the north side.
That, and the position of the hangers (usually towards the center) indicates that 3000 feet of the 06 side was abandoned and the 24 side was lengthened by around 5000 feet.
A newer road, going around the 24 end of the runway, seems to replace the older ones. A topographical map of Guadalcanal Island shows rising terrain bearing 240 from the airport, and flat ground or ocean towards the northeast.


text:
What is the optimum design: elliptical lift distribution or elliptical span loading parameter distribution?
I am getting very confused with these terms. I'm trying to analyse a span loading  distribution given as cl(y)c(y)/ccL. This distribution is almost elliptical but when I plot just cl(y) it changes and I have more cl near the tips due to the reduction in chord.
This made sense from my point how view since the AR is low, the sweep is high and the taper ratio is small. It is like the figures I remember for these cases. However, when I have searched in google I have seen this image:

This wing is pretty similar to mine, and the distribution is similar to my cl(y) distribution. However, the parameter here is cl(y)c(y)/ccL!!
I don't understand completely the difference. When we talk about elliptical distribution for minimum induced drag it is about cl(y) or cl(y)c(y)/ccL? Why is this parameter used and plot and not cl(y)?

Minimum induced drag is reached when the circulation distribution over span is elliptical. Circulation is indeed proportional to the product of the local lift coefficient and the local chord (c$_l \cdot$c). The diagram in your question normalizes this with the overall lift coefficient c$_L$ and the mean chord $\overline{\text{c}}$.
But this is only true if you disregard the structural weight of the wing. Using less load near the tips will reduce the wing root bending moment and, consequently, wing mass, so less lift and wing area are needed in order to carry the same non-lifting load. Even though your drag coeffcient might be higher, your overall drag will be lower.
The optimum will also depend on the size of the wing. Small wings should indeed use an elliptical circulation distribution because their structural weight is low, but in large airplanes a more triangular distribution will be better.
One last word: Please try to avoid the high loads near the wingtips if you want a nice, docile stall behavior.

Thank you so much for your answer! However, I don't fully understand it. As far as I know, L(y)=rhovcirculation(y). Therefore, if you have an elliptical circulation distribution you should have an elliptical lift distribution. This would explain why people use these terms indistinctively.
With this statement I can't see how lift distribution and circulation distribution could be different in shape. Also, I can't understand your statement "Circulation is indeed proportional to the product of the local lift coefficient and the local chord (cl⋅c)".
Thank you again!

Just in case someone have the same question some day: of course cl(y)c(y) is proportional to the circulation. cl(y) is normalized with c(y) and therefore cl(y)*c(y) is l(y)/q.  Lift and circulation distributions have of course the same shape. This must not be confused with cl(y) distribution. The graphs show the effect of some parameters  but this cl(y)c(y) distributions can be changed with twist. If you have an elliptical distribution of lift in a wing with small taper ratio and calculate cl(y), it will have a similar shape to that cl(y)c(y) distribution but that is not the load distribution!

@JavierGarcia: Is there anything still unclear? Should I expand the answer? Let me know if you think it can be improved.

No, everything is clear now! Thank you!


text:
Is low pressure compressor air used to cool turbine blades?

This photo shows LP cooling air is being used to cool turbine blades. the question is, is it possible to use LP air to cool the turbine blades?
would you please guys explain why do they use LP air with HP air to cool the turbine blades?

Bleeding HP costs much more engine performance than LP.

Short answer:
Low pressure compressor air can be used for turbine cooling, but usually it can only be used to cool the low pressure turbine. For the HPT, you need cooling air from the HPC. This is because the cooling air must come from a location in the compressor that is a higher pressure than the part of turbine it is cooling, so it will actually flow into that turbine location.
Longer answer:
Cooling air from the compressor is air that the engine has compressed, so put energy into, but doesn’t get the full energy out of, when it enters the turbine as cooling air. So, you want to use as little as possible, and take it from the early stages of the compressor, rather than the back of the compressor, where even more energy has been put into it.
But.... the cooling air has to be taken from a location within the compressor that is a higher pressure than the turbine location which it is cooling, so that it actually flows into the turbine.
Hence, to reduce the parasitic nature of cooling air, the low pressure turbine is generally cooled with air from the early stages of the compressor, but the high pressure turbine must use cooling air from a sufficiently high source, so it usually comes from the last few stages of the high pressure compressor. As explained, this is necessary for the pressures to be favorable to cause the cooling air to flow. However, aside from meaning it is more parasitic towards the engine, there is another disadvantage of having to use high pressure air for cooling. Pressuring the air causes its temperature to increase. So, the “cooling” air from the exit of the high pressure compressor is hotter than cooling air from the start of the HPC. Because of this, sometimes the cooling air is cooled by a heat exchanger in the bypass duct, because the cooling air from the exit of the HPC is actually quite hot.
Why the picture shows both LP and HP air in the same blade:
In the picture you show, the same blade is being cooled by both LP and HP air. This appears to contradict this answer. If one can use cooler LP air, why use the hotter, more parasitic HP air. I found the source of your image, and the text there doesn’t explain this contradiction. But if you look at the two images attached, you can see that the pressure around a turbine blade is not uniform. There are higher pressure regions especially at the front of the blade, while the pressure at the trailing edge is lower. Since you always need your cooling air to have a higher pressure then the static pressure at the location of your outlet, you need high pressure cooling air at the front where the pressure on the blade is high and you can get away with using lower pressure cooling air at the back since the pressure on the blades trailing edge is low.

Another reason is thermal shock, trying to cool a 1000 degree turbine blade with 100 degree air probably wouldn't end well, that's another reason why they use high stage HP compressor air


text:
How to get the fuel on board reading on UVdatalink via Honeywell ACARS system?
I would like to find out if anyone has encountered the following issue and how was it rectified.
We are currently seeing the following results on UVdatalink for some of our aircraft that have Honeywell ACARS units installed.

We would like to see the following message which we get from Rockwell ACARS units.

Any assistance would be greatly appreciated.

Shouldn't you call Honeywell directly and ask them this question? We don't provide tech support for products, and your question is highly specialized.

@RonBeyer People do ask questions that are this detailed on Stack Overflow......   Granted, I'm not sure we have the kind of user base required to answer this question.  Going to Honeywell is probably going to be the more useful course of action.

@JayCarr each community on stack exchange has its own rules and guidelines. I'm not quite sure tech support is on topic here whereas it is quite sread among stackoverflow.

Thank you guys. I understand that it is a highly specialized question and was being very optimistic for a solution here. We have gone to Honeywell but unfortunately they could not help with the matter.

@ManuH That's actually a good question for Meta (though I wouldn't worry about it yet, considering this is kind of the first question I've seen like this.)  But I'd like to think Aviation.SE could be a resource for plane owners...not sure if that should include commercial.......  So, yeah, a good topic for Meta perhaps.

@JayCarr done. Feel free to comment/edit/answer/....

@ManuH Excellent, looks like it's generating some consensus, that's what I really like to see ;)


text:
Do stratospheric business and military jets use the North Atlantic Tracks?
The North Atlantic Tracks (NATs) are the airways generally used by airliners taking approximate great-circle routes between North America and Europe; due to the wild swings in wind strength and direction over the North Atlantic from day to day and place to place, the NATs are not fixed in position, but, rather, are plotted fresh every morning to allow transatlantic flights as much tailwind (and, thereby, as much groundspeed and as much per-flight fuel efficiency) as possible, and, then, sent to all the airlines preparing aircraft for transatlantic runs.
In contrast, the weather in the stratosphere - the layer of air lying above the tropopause (the upper boundary of the weather-prone troposphere; usually lying between 35 and 50 kilofeet at temperate and polar latitudes, but generally located at a much higher altitude over the tropics) and below the stratopause (the upper boundary of the stratosphere, and the lower boundary of the poorly-known mesosphere; usually located at around 165-180 kft) - is essentially uniform constant from day to day, changing (usually) only slowly, with the seasons.  This removes the raison d’etre for the moving NATs, as no particular stratospheric route is objectively significantly better than any other of comparable length.  For this reason, the Concorde (cruising at flight levels [FLs] 470-600, equating to an altitude of 47-60 kft, give or take a kilofoot or so)1 did not use the NATs, instead following, for open-ocean stratospheric cruise, one of a set of preplotted fixed airways, the locations of which did not change from day to day.
But the Concorde, even when it was still in service, wasn’t the only aircraft to cross the North Atlantic in the stratosphere; business jets (at least the many that are actually capable of doing so) routinely cruise at between FL 450 and FL 550 or so (the exact upper limit depending on the model of bizjet), and some smaller military jets routinely cruise above FL 600 in order to minimise aerodynamic drag on long-distance flights.  These other high-altitude jets would stand to benefit just as much as the Concorde did from being able to use fixed stratospheric airways rather than having to follow the variable and non-stratospheric-aircraft-benefitting NATs; yet, I’ve only ever come across mention of immobile stratospheric routes with reference to the Concorde, and in language that makes it sound like the Concorde was unique in its use of these routes.
So do stratospheric bizjets and military jets have to use the NATs (and, if so, why?), or do they get to use fixed stratospheric routes, like the Concorde did?

1: An aircraft’s flight level is the altitude, in hectofeet, that its barometric altimeter (if properly calibrated, and assuming the aircraft’s pitot-static system is working properly) indicates when set to standard pressure (29.93 inHg).

Question is too longwinded with irrelevant asides. Just ask "Where do the NATs top out, and what routes are available above them?"

@pericynthion, true, however the criteria for an up vote is "Shows research effort, is useful and clear". He definitely scores on the first 2, though could use some effort on the 3rd. OP's questions are a nice change from the dry, "Just the facts, Ma'am" questions we usually get 'round here. Gets my +1 for "kilofeet" alone.

@pericynthion It looks like a great question to me. I don't see how your nitpicking comment helps.

The tropopause is in the low to mid 30s in the mid-north latitudes that the NATs reside (and is in the 20s in the arctic in winter) so an airliner crossing on a NAT at 55 DegN at FL370 will be well into in the stratosphere.
Concorde didn't use the NATs simply because the NATs top out at FL410.  And in any case, the NATs are a large band of parallel tracks several hundred miles wide which are mostly just shifted more northerly or more southerly to accommodate weather patterns and the locations of polar jets etc.  The routings aren't really customized much more than that.
Anyway, because they top out at 410, if your jet can cruise above 410, like a Global or Gulfsteam, you can avoid the NATs and can pretty much file for whatever custom routing suits you that doesn't conflict with someone else.


text:
Why are/were three-pointer altimeters so prone to being misread?
The three-pointer dial altimeter (with a shortest pointer indicating tens of kilofeet, a short pointer indicating kilofeet, and a long pointer indicating hectofeet), the most instantly-recognisable style of analog barometric altimeter, is also, apparently, the type most vulnerable to misreading, with a number of accidents either definitively attributed to, or suspected to have been caused by, pilots misreading three-pointer altimeters (frequently in the form of believing one’s altitude to be ten kilofeet higher than one it actually is).1
Examples include:

A BOAC Bristol Britannia on a test flight crashed on 24 December 1958 while on approach to land; testimony from surviving crewmembers definitively identified a misread altimeter as the cause of the crash.
United Airlines Flight 389 crashed into Lake Michigan on 16 August 1965 after the flightcrew failed to arrest its descent at the altitude to which they had been cleared; the cause of the continued descent into the water was never conclusively determined, but a misread altimeter is strongly suspected.
Iberia Airlines Flight 62 descended below its clearance limit on 4 November 1967 and crashed into a hill in West Sussex; the cause of the crash is not known with certainty, but one of the possibilities is a misread altimeter.

Further, to quote the NTSB report on the crash of UA389:

As a part of this investigation a review of research on altimeter reading problems was conducted.  There are some fifty studies relating to the various altimeter systems of presenting information to pilots which have been conducted since 1947.  One of the most recent studies completed by the Naval Research Laboratory, Washington, D. C. 16/ was conducted to determine the relative effectiveness of various altimeter displays.  These investigations, consisted [sic] of laboratory research at NRL and flight test [sic] at a nearby naval air facility, were in support of a Department of Defense program.  Four types of altimeter presentations, the counter-pointer (CP), counter-drum-pointer (CDP), drum-pointer (DP) and three pointer (3P) were compared in a series of laboratory experiments measuring reading time and accuracy with both pilots and non-flying enlisted men.  The same instruments were also compared by the pilots in flight, using the questionnaire technique to elicit pilot judgments.  The results of the various phases of laboratory research showed that the three-pointer altimeter consistently ranked poorest.  The pilot preference in the flight tests was overwhelmingly in favor of one of the other types of altimeter.  In all objective measurements taken, the three-pointer altimeter ranked the poorest among the instruments tested.  The finding that the three-pointer was poorest proved to be statistically reliable in every study in which tests of statistical significance were applied.  In a preceding part of this study 17/ the mean exposure time and number of errors on each altimeter was recorded for the two test groups, one consisting of pilots, and the other of non-pilot enlisted men.  In 1080 readings of the four altimeters by 18 pilots the mean exposure time in seconds was 0.80 for the CP, 0.84 for the CDP, 1.38 for the DP, and 2.28 for the 3P.  The number of reading errors was 7 for the CP, 14 for the CDP, 26 for the DP, and 80 for the 3P.  The same data collected during 420 reading trials using 7 non-pilot enlisted men resulted in mean exposure times of 0.85 for the CP, 0.86 for the CDP, 1.50 for the DP, and 2.93 for the 3P.  The number of errors by this group were 5 for the CP, 2 for the CDP, 10 for the DP, and 52 for the 3P.  The mean number of errors made on each altimeter for pilots was 0.39 CP, 0.78 CDP, 1.44 DP, and 4.44 3P, and for enlisted men 0.71 CP, 0.29 CDP, 1.43 DP, and 7.43 3P.  [Pages 24-25 (paper report)/28-29 (PDF).]

This is quite surprising, given that the three-pointer altimeter display works on the same principle as an analog clock face, and even nowadays (with the proliferation of digital clock displays), the vast majority of people can rapidly and accurately read the time displayed on an analog clock; back in the 1960s (before the digital clock was a consumer item), practically every literate adult would have been able to do this.  It shouldn’t be hard to translate those skills into reading altitude rather than time, so that, instead of (say) 3 hours (shortest pointer) 25 minutes (short pointer) 32 seconds (long pointer) past midnight, you have (say) twenty- (shortest pointer) -five thousand (short pointer) seven hundred (long pointer) feet above mean sea level.
What gives?

1: It should be noted, however, that other altimeter styles are not immune to being misread; see, for instance, AA320 and SA228, both involving the misreading of drum-type altimeters.

Related

Perhaps the claim that people can "rapidly and accurately read the time" is rather optimistic, esp. when seconds must be read too (3 hands). I wonder if there was a proper research into that.

Actually, in altimeter, the pointers are the long hand for hundreds, short somewhat trapezoidal hand for thousands and the triangle or point running on the outside for tens of thousands. And the triangle is of course often at the end of a very thin hand, so it makes it the longest, but it is the highest order, not lowest.

"the vast majority of people can rapidly and accurately read the time displayed on an analog clock". You obviously do not spend much time around many US school children. It seems very few of them can read an analog clock. So few that, instead of teaching them how, many US schools are removing them from their buildings. (Seems bassakwards to me, but that's what they seem to be doing.)

If you are 10K' off in interpreting your altimeter then there had to have been a severe breakdown in situational awareness that instrument design would be unable to fix.

The altitude is quarter to 10,000. No wait, one minute before 8,000. Let's see, 5 divisions between 7 & 8 so altitude is 7,800 and zero hundreds.
No it isn’t. From Jan Hudec’s comment:

The thin hand is a bit strange—normally it is just the triangle running on the outside or the hand itself is black—but the triangle means it is the 10,000 one, so it should be 780, not 7,800. The long straight hand is always hundreds, the short trapezoidal is always thousands and the triangle on the outside is always tens of thousands. Still clearly demonstrates just how confusing it can get indeed.

It is a design issue: it looks like an analogue clock but works differently. The long thin one denotes the major units, not the minor one of the second hand. Units can be 10,000, 1,000 or 100 foot. The thousands are probably most intuitive, the 10,000s and the 100 are not.
Perhaps a different story in China, Japan and Korea, where there is a dedicated word for 10,000

On top of this, the cockpit is not always an easy environment - you're bouncing in turbulence, avoiding deadly weather, managing a low fuel state... mistakes are easier to make when you've got all that too.

The link is dead...

@sweber thanks, fixed it.

The thin hand is a bit strange—normally it is just the triangle running on the outside or the hand itself is black—but the triangle means it is the 10,000 one, so it should be 780, not 7,800. The long straight hand is always hundreds, the short trapezoidal is always thousands and the triangle on the outside is always tens of thousands. Still clearly demonstrates just how confusing it can get indeed.

@JanHudec: I was under the impression that the long straight hand was hundreds, the short trapezoidal hand was thousands, and an even shorter, cross-shaped hand was for tens of thousands...

@Sean, I haven't seen that many different real instruments, but judging from images and simulators the triangle on the outside seems to be most common for tens of thousands. There is not a complete consistency though, so some instruments may use very short cross-shaped hand or other shapes. The other hands are almost standard, but there are probably some exceptions too.

I think the discussion in the comments is the answer! It demonstrates just how confusing this is!


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What would the call sign of a non-standard aircraft carrying the US President be?
So, it's well known that the government planes operated for the US President are called Air Force One; less well-known is that this is a call sign given to any Air Force aircraft that carry the president, and that if the President rides in an aircraft owned by another branch of the US Armed Forces, they would become known as Marines One, Navy One, or Army One, while any civil aircraft that carries the US President becomes Executive One.
What about aircraft that are exceptions to the normal FAA rules for aircraft licensing, however? If the US President was riding in (or piloting) a glider, ultralight, or autonomous drone taxi, what would their call signs be, if any?

"any civil aircraft that carries the US President becomes Executive One" Doesn't this already answer your question? Why wouldn't a glider, ultralight, etc. qualify as "Executive One"?

This question is unanswerable in its present state. A glider is a certified and registered aircraft in the US. Ultralights are not register aircraft in the US. And, they are not typically in contact with ATC. Autonomous drone taxis have yet to receive FAA certification, airworthiness, and registration. So, the best manufacturers can do with drones at this time is to fly them unmanned or on test flights.
The bigger question is whether the Secret Service would allow the president to pilot any aircraft. Let alone fly in a non-certified, unregistered aircraft. After all, they do not allow the president to drive a car while he is in office.

All the President needs to fly a plane or drive a car is the appropriate licence. The Secret Service works for the President, not the other way around. All the Secret Service can do is make recommendations to the President, and remind the President of their diminished ability to protect the President if their recommendations aren't followed.

@RossRidge: I get the impression that most Presidents take those recommendations very seriously, though.  Good point that one could choose not to follow a recommendation, but generally they don't want to antagonize people who's literal job is to keep them safe.  Still, exceptional circumstances can happen, especially in fiction (movies like Air Force One, or you could imagine that having everyone hop into some conveniently-present ultralight aircraft would be safer than staying somewhere when a tsunami was coming, or whatever other Hollywood situation.)

@RossRidge - Actually, Secret Service works for Homeland Security. Presidents work for the people. Each has requirements and restrictions for their job. Presidents Biden, Obama, Bush 43, Clinton (and wife), and Reagan have all commented at various times how they are not “allowed” to drive even after they leave office. Let alone fly an airplane. The last president to drive was reportedly LBJ. From my understanding, although there is no actual law that says POTUS can not drive, there is one that says POTUS can not refuse the protection of Secret Service. And, their job is to protect POTUS.

@DeanF The fact it's the Secret Service's job to protect the President doesn't give them the ability to prevent the President from driving a car or flying a plane. Nor does it give them the ability to stop the President from staging a impromptu photo op in front of a church during a protest. The Secret Service just has to do their job to protect the President as best they can under the circumstances.

@RossRidge - It is sort of like the rule that POTUS can not manage their own investments. Even if they have an investment license from the SEC. There are ways around it, I am sure. Just like Reagan and Bush driving a bunch of old trucks on their private ranches. But, they were not allowed on the road. Now, could they have been petulant about it and driven anyway? Yes. Did they? No. There is little upside and plenty of downside for doing it. Actors and professional athletes go through the same thing during the sports season or while contracted to a movie or TV show.

The whole trip was pretty much a stunt, but #43 reportedly manipulated the controls of Navy 1 on the flight out the carrier; albeit not as PIC.  Reportedly he wanted an F18, but the secret service bargained him down to an S-3B with room for a second pilot and one agent.

@RossRidge - By the way. Both President Bush 41 and 43 were pilots.

So when Trump did that photo op where he got into the cab of a truck and tooted the horn, he was OK as long as he didn't put it into gear.

Driving a car opens a huge room of liability to follow. Imagine how bad PR it would be if ex-president drive a car and kill a poor dog.

@Barmar - The key words are “photo op”. I did not see this particular photo op. If it was an 18-wheeler, I doubt Pres. Trump (or any other POTUS) knows how to drive it. If he was on private land on a closed course, it would not matter (see previous comments about Reagan and Bush). If he were on public roads, he would need to be properly licensed for that size of vehicle (I seriously doubt he is). So, yes. He was OK.


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What regulations govern UAV lights in Australia?
I'm a member of a student aviation club at my university, and we're working on a UAV. The club member working on our drone's power supply system has designed a system that uses externally-visible LED lights for displaying system errors to allow for visual inspection of the functionality of our systems, and to allow for us to determine if something goes wrong in flight by looking at it from the ground.
However, I'm uncertain if this would comply with CASA regulations regarding aircraft lights; if it isn't allowed, the current plan is to simply cover the them with a cowling prior to takeoff so that they won't be externally visible. Doing a Google search, I found this page on CASA's website detailing requirements for mandating certain navigation lights, but it doesn't say anything about restricting them. It seems fairly likely that red, green, or white lights would be prohibited to prevent confusion with the navigation lights, but would amber lights be allowed?

Does your question concern only night flight, or daytime flight as well?

For night time flight you need to comply with https://www.casa.gov.au/drones/reoc/flying-at-night . Lights shouldn't matter during day time, but I cannot substantiate that.

@bogl Yes, it’d be flying during the day as well.


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Does pilot-controlled lighting work if you are interrupted by another voice transmission?
This question is in the context of pilot-activated runway lighting, which involves clicking the microphone e.g. 7 times to turn on the lights at night. I was wondering what happens when you are clicking the radio button, which typically takes several seconds, and suddenly someone else starts transmitting on the same frequency while you are still clicking? Does that mean your attempt has failed, since the other person's transmission is often continuous? To what extent is the receiver on the ground able to discern between the two signals? Does it depend on our relative proximity to the airfield? Or is this nothing I should worry about?

From a technical standpoint, it's extremely difficult to tell two different signals on the same frequency apart. So, if someone steps on you while you're trying to turn the lights on, then the ground equipment can't tell the difference between their radio and yours. So, unless the other radio is far enough away that it doesn't register on the PCL receiver at all*, it won't activate. You'll just have to try again when the other pilot's done talking.
In general, it's usually nothing to worry about, because the other pilot(s) nearby will hear you clicking, and know to avoid using their radio until you're done.
* Many airports turn the sensitivity on their PCL down so that distant planes on the same frequency won't interfere with a local pilot trying to land.

Why wouldn't the mic clicks still register while the other pilot is talking?

@MichaelHall Because the signal level never falls below the detection threshold, so the receiver sees it as one continuous signal. That's what I was talking about when I said it was difficult to tell the signals apart, they get mixed together at the antenna.

Gotcha, thanks!


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Does rolling affect the thrust of a propeller?

If a plane with a propeller, e.g., a Spitfire, does a constant barrel roll in the same direction as the propeller, will the plane travel faster as impractical?

"will the plane travel faster as impractical" ??? Can you re-word that? "impractical" is not the correct word here, I'm guessing you want to know if the roll rate is helped/hindered by the torque effects of the engine?

for a propeller rotating at 2400RPM = 40 rev/second and a roll rate of 0.3 rev/second, the roll rate will not measurably affect the prop rotation speed and the plane will neither travel faster (if the rates are additive) nor slower (if the rates are opposite).

Well, it's almost a difference of a whopping 1%

I do not think the airspeed indicator would resolve that difference. this is why I said "measurably affect".

The plane will fly slower
If the plane is constantly rolling, then both ailerons are extended into the slipstream, causing drag.
During approximately 1/2 the roll, the plane is in knife-edge flight.  In that condition, the aircraft's fuselage is providing lift.  It's a very inefficient way to fly and usually involves a significant nose-high attitude (more drag).

The fuselage produces lift when the aircraft fuselage angle-of-attack (AOA) angle is non-zero. And any lift that it produces would be in the direction of this AOA. i.e., if the aircraft maintains a zero sideslip angle, will always be perpendicular to the plane of the wings. The aircraft roll angle or attitude has nothing to do with it.

@CharlesBretana What happens when the aircraft has rolled 90 degrees and the wings are no longer contributing to lift?

You are confused by the multiple and overly simplistic uses of the term Lift. Lift is not only "the force that holds the aircraft up". It is "the force generated by the flow of airstream". - this is especially relevant if you wish to discuss the Lift produced by parts of the airframe other than the wings. So, to answer your question, the Wings are always producing Lift, even if you are upside down. In 90 degrees of bank all this lift is turning the aircraft sideways, changing its heading.

If the aircraft AOA is zero, the "Lift" points backwards, parallel to the flight path, and slows the aircraft down without changing its flight path direction. One "artificial" definition of Lift (redefined to allow us to split it up into lift and drag, is to define it as only that component of the total force that is perpendicular to the flight path, and the drag is that component parallel to he flight path. Another common redefinition (which you are assuming) is to use the horizon as the frame of reference or orientation. But all these are artificial, and used only for convenience.

And to answer the last question (implied) in your comment, at 90 degrees of bank, the aircraft will not maintain level flight, it would fall, since (as you correctly surmise), the lift which is still being generated by the wings is not pointed up, and so can no longer keep the aircraft from falling.

@CharlesBretana: Are you again redefining English? "Lift" pointing backwards is not called lift but drag. Do yourself and us a favor and use proper terms around here.

@Peter, Yes, conventionally, you are correct. I apologize. Lift is the component of the total aerodynamic force that is perpendicular tot he flight path (or the relative airstream). I guess my point (that this is an artificial mathematical abstraction created for convenience in visualization and for computation), is obscured by my misuse of the word.

There as an interesting point here underneath all the arguments.  About what axis is the plane rolling?  If it is initially flying at a high angle-of-attack and then rolls around the longitudinal axis, this automatically converts angle-of-attack into sideslip by the time the roll has progressed to 90 degrees.  At 180 degrees, if the roll is still progressing around the same axis, the inverted wing will be placed in a negative angle-of-attack, creating skyward lift.


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Why does the Boeing 737 have a pitot tube on the tail?
From this link:

Can anyone explain why the Boeing 737 has a pitot tube for the elevators? Wouldn't the airspeed be measured the same from the front of the airplane and the tail? Is the airspeed measured at the tail view-able from the cockpit?

There are five sets of pitot tubes on the 737, organized into two groups, the pitot tubes on the nose are used for airspeed measurements, independent for the pilot and copilot and one as a backup. There are two pitot tubes on the tail that are for the "elevator feel and centering unit" (pdf; page 8).

The elevator feel computer provides simulated aerodynamic forces using airspeed (from the elevator pitot system) and stabilizer position. Feel is transmitted to the control columns by the elevator feel and centering unit.

The airspeed may not be the same due to shielding in high angle-of-attack situations. I do not believe that the airspeed from the pitot tubes in the tail is visible (or useful) to the pilots. There is a warning that appears in the cockpit when the sensor fails.

The airspeed around the airfoil should be significantly faster than the indicated airspeed for the pilots... should work well if it simulates control forces since that is the speed that the flap surface "sees"

The 2 pitot tubes on the dorsal fin provide pitot pressure (raw airspeed data) to the elevator feel and centering unit to provide artificial feel to the control column.  The faster the aircraft flys the more force it will take to move the elevator fore and aft because at higher airspeeds less control inputs are required.  This force (artificial feel) helps prevent the pilot from over controlling the aircraft at higher speeds.  There are two because one provides input to the "A" side and the other for "B" side.  The elevator feel computer (it's a mechanical computer NOT an electrical one) compares the output hydraulic pressures and if there is a difference it will illuminate a light on the P5.

Hm, I've always thought that the hydromechanical system in use can multiply the force both ways and therefore provides real feel according to the actual aerodynamic forces on the control surfaces.


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Are Cargo Holds Pressurized, Non-pressurized, or vacuumed?
I watched an episode of Air Crash Investigation, which depicts a plane catching fire due to it shipping many expired oxygen generators for planes. The narrator says that a fire in the cargo hold is very unlikely, as the cargo hold is vacuumed, to prevent fire; but however in this situation, the oxygen generators were actually generating oxygen so the vacuum was futile. I also know they carry pets in the cargo hold. So are the cargo holds pressurized, non-pressurized, or vacuumed? Or does it depend on the carrier, or aircraft?

And secondly, if a fire has a heat source and something to burn but is deprived of oxygen it can smolder for a long time and as soon as the hold is opened it can catch quick

And secondly, if a fire has a heat source and something to burn but is deprived of oxygen it can smolder for a long time and as soon as the hold is opened it can catch quick

The cargo hold is indeed vacuumed, using a vacuum cleaner, regularly.
It is however NOT drawn vacuum (so having all air removed).
Some cargo holds may be pressurised, some are not. Depends on the aircraft and the requirements of the cargo (life animals want a pressurised cabin of course, just to give one example).


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Do overland long-haul flights have something similar to SELCAL?
This question will be regarding airlines, flying high altitudes.
As with short flights, you are in contact with local controllers, then get passed off to enroute and local again.
I am familiar with SELCAL, where you do not have to listen to HF Radio jumble for your 7 hour oceanic flight.
However, what if you are flying for example Dublin to Hong Kong, not crossing oceans. Will you be in contact with ATC the whole time, and the pilots on duty always having to wear headsets just in case if ATC wants to find them? Or would there be a system similar to SELCAL where they will buzz the aircraft only when they need them?
(skyvector.com)

Remember that contact with ATC isn't only about "when ATC needs you" or even when you need ATC; it's also about situational awareness. Compare for example How do pilots/controllers deal with overlapping transmissions on the radio? I know there have been several more answers discussing the importance of the situational awareness resulting from other aircrafts' air-to-ground chatter, but that's the one I could find quickly.


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What differentiates the entry procedures for spins and snap rolls?
Both a spin and a snap roll are aerobatic manoeuvres involving stalling one of an airplane's wings earlier and more deeply than the other.  Both are entered by pitching up sharply to stall the airplane, and applying a hard rudder input to yaw the airplane sideways and ensure that one wing stalls before the other, causing that wing to drop and the airplane to roll off to the side.  What happens next differs between the two:

In a spin, the increase in drag on the wing that stalls first causes the airplane to enter an extremely fast yaw (rotation about a vertical axis1) in the direction of said wing, keeping it more deeply stalled than the other wing; the high yaw rate centrifuges the airplane's mass away from the spin axis, preventing the airplane from rolling beyond the bank angle induced by the initial spin-entry yaw input.
In a snap roll, it's the decrease in lift on the first-to-stall wing which is important (rather than the increase in drag), causing the airplane to enter an extremely fast roll (rotation about the aircraft's longitudinal axis) into the first-stalled wing; unlike with a spin, the airplane's inherent directional stability prevents the initiating yaw input of a snap roll from producing any persistent yaw.

Thus, the same type of entry procedure can apparently result in two wildly different outcomes: a spin, with an extremely-high yaw rate but no significant roll rate, or a snap roll, with an extremely-high roll rate but no significant yaw rate.
How does one differentiate a spin-entry procedure from a snap-roll-entry procedure?  What determines whether the sudden pitch-up and hard rudder input produces a spin, or whether it generates a snap roll instead?

1: This axis usually is not coincident with the vertical axis of the airplane as a whole (that passing through the aircraft's center of mass), although it can be.

A snap roll isn't a roll.  It's an "enhanced" spin, done horizontally.
A normal spin entered from level flight at 1G stall speed has an initial roll component along with the yaw component as the wing drops at the stall.
The same thing happens in a snap roll, except that the spin is entered with an accelerated stall, well above level flight stall speed and with power, with both the roll and yaw initiation enhanced with aggressive rudder and aileron input to get the horizontal auto-rotation roll-yaw going as fast as possible.
Because the snap is done in an accelerated stall, the stall can be broken immediately with forward stick, and aggressive opposite control inputs have to be used to overcome inertia to stop the snap roll.  You initiate the left snap with the hard stick back/left and left rudder, and recover with hard stick forward/right and right rudder when about 3/4s of the way around.
You don't actually need aileron; airplane can snap roll quite nicely with no aileron, just rudder, it'll just be a bit slower.  A pilot can enter a snap roll accidently just from mishandling, say pulling too hard in a turn and inducing an accelerated stall with the ball off center, and if the airplane has a sharp stall break, around you go, and before you've figured out what happened you're already right side up again.
If you hold the control inputs and keep the snap-roll going, you eventually arc down until you're snap-rolling vertically, or spinning the old fashioned way.

Just a note -- snap roll can be performed in any direction, not only horizontal. There is even vertical downwards snap (9.9.10 in Aresti [https://www.fedach.cl/images/pdf/reglamento/FAI-Aresti-Aerobatic-Catalogue.pdf ], I had to look it up, but yes, it's there :) ). But the entry will be (in contrast to spin) from flying nose down already, while spin starts slow horizontal flight and nose drops first after the stall.

Exactly as John described, the main difference is speed. Or, more correctly, speed compared to what is necessary to maintain the current flight path.
Spin (at least as a competition figure) can be entered only from slow horizontal flight. Approaching 1G stall speed, airplane will eventually lose ability to generate enough lift for maintaining flightpath and while holding elevator all the way back, nose will drop in motion which is usually associated with "common" stall. You actually do not need any abrupt input to make it into spin (well, depends quite a lot on particular airplane, but generally it should be so). Just simply approach 1G stall speed with a bit out of coordination and keep pushing on the "wrong" pedal while nose is dropping down and ... done, you are spinning down. You can make transition more abrupt to make it look nicer, but it is not strictly necessary for entering the spin.
How much yaw- or roll-rotation you get depends on many parameters of particular airplane, c.g. and even duration of spin. There are airplanes (and c.g. positions in them) which will spin nose down with more roll-motion and some which will hold nose closer to horizon and performing more yaw-like rotation. And with number of turns (as speed builds up etc.) the characteristic can even change. Check flat-spin too.
In snap roll OTOH initial flight path is more-or-less maintained during the whole maneuver and you should not be anywhere close to stall-like dropping the nose. This does not need to mean high speed, if you fly, for example, snap at the top of looping (avalanche) where flight path asks for very low gee, or vertical snaps, you do not need much airspeed to maintain the c.g. trajectory.
What you need is an abrupt elevator input (so it will not change flight path but only AoA of wins without affecting overall movement through air too much). Actually, it is possible to perform negative snap from positive flight and vice versa (that is, to stall wing for snap roll in the opposite direction than it is generating lift for sustained flight when entering the maneuver). In this way, with help of rudder input you get one wing over its critical AoA and resulting lift difference will push airplane through the roll.

The Snap or Flick Roll certainly is a roll. The aircraft rotates around the longitudinal axis. This is by definition a roll.
It is just not an “aileron roll”, although some aileron can be used in the normal way to assist the “rudder roll”.
It differs from a spin in that neither wing is stalled prior to the application of rudder. When rudder is applied the resulting yaw stalls one wing only (or one side in a biplane). The resulting large difference in lift causes the rapid roll.
In a spin both wings (or sides) are stalled initially and the application of rudder (and other controls) varies how much more or less stalled the two sides are.

It is the timing that makes for a good flick roll. The aeroplane is given a sudden increase in angle of attack with elevator but not quite enough to stall the wings. At that point full rudder will stall one side. The elevator should then be relaxed forward of neutral (in a positive flick) so you don’t lose a lot of energy. The rotation is increased by doing this. The flick is stopped by full opposite rudder at the right moment with only a tad more forward stick. Half flicks, which I have yet to try, require the technique to be modified.